Everything had been fine here, until I wanted my LogiTech USB-mouse: Added some (hopefully) correct modules in menuconfig and recompiled; even saved the old bzImage:
# cp bzImage bzImage.okaywithoutUSB
Now eth0 doesn't start; as 'M' it has some 10 unresolved symbols; as 'X' it doesn't either. Even
# mv bzImage.okaywithoutUSB bzImage
does not help and brings a similar result, though it had started previously.
1. - of course, any idea what to do; my menuconfig looks good to me
2. - how can I really save a working kernel so that it can be re-instated later?